1. Field of the Invention
The invention relates to the field of sawing semiconductor wafers from a boule and more particularly to saw feed control systems.
2. Prior Art
Silicon wafers are sliced from monocrystalline silicon boules using wafer saws, lapped to remove saw damage such as gouges and surface unevenness, and etched to remove any surface damage caused by or not removed by the lapping, all prior to beginning device fabrication.
If as-sawn wafers could be made flat enough and smooth enough that the lapping step could be eliminated, wafer production would be simpler and more economical. In order to achieve this goal, improvements are needed in the present wafer sawing process to provide much smoother as-sawn wafers.
Semiconductor wafer slicing saws are known which feed the saw blade into the boule at a constant linear displacement per unit time throughout the sawing of a wafer from a boule. Such systems are either inefficient or harmful because they cannot be fully efficient near the edge of the boule where the initial slicing begins without feeding the saw blade too fast ner the center of the boule.
U.S. Pat. No. 3,039,235 to Heinrich discloses a saw control system designed to eliminate this dichotomy by varying the rate of feed of the saw blade in a predetermined pattern which causes the material of the boule to be removed at a constant rate (in grams per minute) independent of the degree of penetration of the saw into the boule. This system is more efficient than constant feedrate systems. However, we have now found that, in fact, this system does not consistently provide either optimum efficiency or the highest quality wafers.
These prior art saw control systems provide no accurate, reproducible means for determining when the cutting efficiency of the saw blade has diminished to the point where the saw blade should be dressed, or replaced if dressing is not effective.